Effective Surface Charge Density Research Articles

Bridging Ionic Current Rectification and Resistive-Pulse Sensing for Reliable Wide-Linearity Detection.

As two mainstream ionic detection techniques, ionic current rectification (ICR) suffers from large fluctuations in trace level detection, while resistive-pulse sensing (RPS) encounters easy clogs in high-concentration detection. By rationally matching the nanopore size with the DNA tetrahedron (TDN), this work bridges the two techniques to achieve reliable detection with wide linearity. As a representative analyte, miRNA-10b could specifically combine with and release TDN from the interior wall, which thus induced the simultaneous generation of distinct ICR and RPS signals. The ICR signals could be attributed to the balance between the effective orifice and surface charge density of the inner wall, while the RPS signals were induced by the complex of miRNA-10b and TDN passing through the nanopore. Such an operation contributed to a wide detection range of 1 fM-1 nM with a good linearity. The feasibility of this method is also validated in single-cell and real plasma detection.

A Rolling-Bead Triboelectric Nanogenerator for Harvesting Omnidirectional Wind-Induced Energy toward Shelter Forests Monitoring.

Shelter forests (or shelter-belts), while crucial for climate regulation, lack monitoring systems, e.g., Internet of Things (IoT) devices, but their abundant wind energy can potentially power these devices using the trees as mounting points. To harness wind energy, an omnidirectional fluid-induced vibration triboelectric nanogenerator (OFIV-TENG) has been developed. The device is installed on shelter forest trees to harvest wind energy from all directions, employing a fluid-induced vibration (FIV) mechanism (fluid-responding structure) that can capture and use wind energy, ranging from low wind speeds (vortex vibration) to high wind speeds (galloping). The rolling-bead triboelectric nanogenerator (TENG) can efficiently harvest energy while minimizing wear and tear. Additionally, the usage of double electrodes results in an effective surface charge density of 21.4µCm-2 , which is the highest among all reported rolling-bead TENGs. The collected energy is utilized for temperature and humidity monitoring, providing feedback on the effect of climate regulation in shelter forests, alarming forest fires, and wireless wind speed warning. In general, this work provides a promising and rational strategy, using natural resources like trees as the supporting structures, and shows broad application prospects in efficient energy collection, wind speed warning, and environmentally friendliness.

Multifunctional graphene heterogeneous nanochannel with voltage-tunable ion selectivity

Ion-selective nanoporous two-dimensional (2D) materials have shown extraordinary potential in energy conversion, ion separation, and nanofluidic devices; however, different applications require diverse nanochannel devices with different ion selectivity, which is limited by sample preparation and experimental techniques. Herein, we develop a heterogeneous graphene-based polyethylene terephthalate nanochannel (GPETNC) with controllable ion sieving to overcome those difficulties. Simply by adjusting the applied voltage, ion selectivity among K+, Na+, Li+, Ca2+, and Mg2+ of the GPETNC can be immediately tuned. At negative voltages, the GPETNC serves as a mono/divalent ion selective device by impeding most divalent cations to transport through; at positive voltages, it mimics a biological K+ nanochannel, which conducts K+ much more rapidly than the other ions with K+/ions selectivity up to about 4.6. Besides, the GPETNC also exhibits the promise as a cation-responsive nanofluidic diode with the ability to rectify ion currents. Theoretical calculations indicate that the voltage-dependent ion enrichment/depletion inside the GPETNC affects the effective surface charge density of the utilized graphene subnanopores and thus leads to the electrically controllable ion sieving. This work provides ways to develop heterogeneous nanochannels with tunable ion selectivity toward broad applications.

The Peculiarities of Electret Effect in Corona Electrets Based on Nonpolar and Polar Polymers With Dispersed Montmorillonite

Polyethylene (PE) and polylactide (PLA) films with 2 and 4 vol% of montmorillonite (MM) are compared to each other and to nonfilled samples. The electret characteristics of initial and composite films (values of the surface potential, effective surface charge density, and electric field strengths) are determined. Filling PE and PLA with MM improves electret properties due to changes in heterogeneous structure.

Achieving Ultrahigh Effective Surface Charge Density of Direct-Current Triboelectric Nanogenerator in High Humidity.

As an emerging energy-harvesting technology, the triboelectric nanogenerator (TENG) is considered a powerful driving force toward the new-era of Internet of Things and artificial intelligence, but its output performance is dramatically influenced by environmental humidity. Herein, a direct current TENG (DC-TENG) based on the triboelectrification effect and electrostatic breakdown is reported to address the problem of output attenuation in high humidity environments for the conventional TENGs. It is found that high humidity not only enhances the sliding triboelectrification effect of hydrophobic triboelectric materials, but also promotes the electrostatic breakdown process for DC-TENG, thus contributing to the improvement of DC-TENG output. Furthermore, taking poly(vinyl chloride) film as the friction layer, the effective surface charge density of DC-TENG with microstructure-designed electrode achieves a milestone value of ≈2.97 mC m-2 under 90% relative humidity, which is almost 1.42-fold larger than that under 30% RH. This work not only establishes an effective methodology to boost the output performance of TENG in a high humidity environment, but also establishes a foundation for its practical applications in large-scale energy harvesting.

Modifying surface charge density of thermoplastic nanofluidic biosensors by multivalent cations within the slip plane of the electric double layer

Thermoplastic nanofluidic devices are promising platforms for sensing single biomolecules due to their mass fabrication capability. When the molecules are driven electrokinetically through nanofluidic networks, surface charges play a significant role in the molecular capture and transportation, especially when the thickness of the electrical double layer is close to the dimensions of the nanostructures in the device. Here, we used multivalent cations to alter the surface charge density of thermoplastic nanofluidic devices. The surface charge alteration was done by filling the device with a multivalent ionic solution, followed by withdrawal of the solution and replacing it with KCl for conductance measurement. A systematic study was performed using ionic solutions containing Mg2+ and Al3+ for nanochannels made of three polymers: poly(ethylene glycol) diacrylate (PEGDA), poly(methyl methacrylate) (PMMA) and cyclic olefin copolymer (COC). Overall, multivalent cations within the slip plane decreased the effective surface charge density of the device surface and the reduction rate increased with the cation valency, cation concentration and the surface charge density of thermoplastic substrates. We demonstrated that a 10-nm diameter in-plane nanopore formed in COC allowed translocation of λ-DNA molecules after Al3+ modification, which is attributed to the deceased viscous drag force in the nanopore by the decreased surface charge density. This work provides a general method to manipulate surface charge density of nanofluidic devices for biomolecule resistive pulse sensing. Additionally, the experimental results support ion-ion correlations as the origin of charge inversion over specific chemical adsorption.

Variable Direct Electromechanical Properties of As-Electrospun Polystyrene Microfiber Mats with Different Electrospinning Conditions.

As-electrospun microfiber mats comprising atactic polystyrene (aPS), a low-cost commodity polymer, have demonstrated beneficial electromechanical properties. However, the variability of the electromechanical properties of fiber mats produced using different electrospinning conditions has not been investigated. Therefore, herein, the direct electromechanical properties of aPS fiber mats produced using different deposition times (tdep) and electrospinning voltages (VES) are investigated. The resulting apparent piezoelectric d constant (dapp) of the fiber mats demonstrates a specific peak value for tdep as high as ~1600 pC N−1 under 1-kPa pressure application after ~0.2-kPa pre-pressure application, although the dapp of the fiber mats produced with some conditions is nearly zero pC·N−1. Furthermore, the peak position of dapp with tdep is fundamentally determined with σEff0/YD(h-hpre) [σEff0: effective surface charge density, YD(h-hpre): secant modulus of elasticity]. Charge distribution models for fiber mats with different tdep are established. The models explain the characteristics of the significant changes in YD(h-hpre) and σEff0 with tdep. These findings provide significant directions for the production of fiber mats with improved direct electromechanical properties.

Interactions between conducting surfaces in salt solutions.

In this work, we simulate interactions between two perfectly conducting surfaces, immersed in a salt solution. We demonstrate that these forces are quantitatively different from those between (equally charged) non-conducting surfaces. There is, for instance, a significant repulsion between net neutral surfaces. On the other hand, there are also qualitative similarities, with behaviours found with non-conducting surfaces. For instance, there is a non-monotonic dependence of the free energy barrier height, on the salt concentration, and the minimum essentially coincides with a flat profile of the apparent surface charge density (i.e. the effective net surface charge density, some distance away from the surface, when accounting for ion neutralization), outside the so-called Stern layer. These conditions can be described as "perfect surface charge neutralization". Despite observed quantitative differences, we demonstrate that it might be possible to mimic a dispersion containing charged colloidal metal particles by a simpler model system with charged non-conducting particles, using modified particle-ion interactions.

Strain-Induced Spatially Resolved Charge Transport in 2H-MoTe2

Strain engineering offers unique control to manipulate the electronic band structure of two-dimensional (2D) materials, resulting in an effective and continuous tuning of the physical properties. Ad hoc straining of 2D materials has demonstrated state of the art photonic devices including efficient photodetectors at telecommunication frequencies, enhanced-mobility transistors, and on-chip single photon sources, for example. However, in order to gain insights into the underlying mechanism required to enhance the performance of the next-generation devices with strain(op)tronics, it is imperative to understand the nano- and microscopic properties as a function of a strong nonhomogeneous strain. Here, we study the strain-induced variation of local conductivity of a few-layer transition metal dichalcogenide using conductive atomic force microscopy. We report a strain characterization technique by capturing the electrical conductivity variations induced by local strain originating from surface topography at the nanoscale, which allows overcoming limitations of existing optical spectroscopy techniques. We show that the conductivity variations parallel the strain deviations across the geometry predicted by molecular dynamics simulation. These results substantiate a variation of the effective mass and surface charge density by 0.026me and 0.03e for every percent of uniaxial strain, respectively, derived using band structure calculation based on the first-principles density functional theory. Furthermore, we demonstrate modulation of the effective Schottky barrier height by quantifying its alteration originating from a gradual reduction of the conduction band minima as a function of tensile strain. Such spatially textured electronic behavior via surface topography-induced strain variations in atomistic-layered materials at the nanoscale opens up exciting opportunities to control fundamental material properties and offers a myriad of design and functional device possibilities, such as for electronics, nanophotonics, flextronics, or smart cloths.

Effect of pH on Effective Slip Length and Surface Charge at Solid-Oil Interfaces of Roughness-Induced Surfaces.

Featured Application Drag reduction in many applications of micro-/nano-fluidic channel. In the development of micro/nano fluid control systems, fluid resistance has always been one of the key factors restricting its development. According to previous studies, it is found that the boundary slip effect of the solid-liquid interface can effectively reduce the resistance of the microfluid and improve the transport efficiency of the microfluid. The boundary slip length is mainly affected by surface wettability, roughness, and surface charge density. Among them, the influence mechanism of surface charge density on the boundary slip is the most complicated, and there is a lack of relevant research, and further investigation is needed. In this paper, we present research on quantification of effective slip length and surface charge density, where the roughness effect is considered. The electrostatic and hydrodynamic force data obtained from atomic force microscopy (AFM) measurements were fitted and processed for comparative analysis. We obtained the variation of surface charge density and effective slip length when different oleophobic surface samples were immersed in ethylene glycol with different pH values. The effect of pH on the surface charge density and effective slip length was investigated by their variations. The mechanism of the effect of pH on the surface charge density was discussed. The experimental results show that in the ethylene glycol solution, no matter whether the pH value of the solution increases or decreases, the charge density of the surface with the same properties decreases, and the effective boundary slip length also shows a downward trend. In deionized water, the surface charge density and effective boundary slip length decreases with the decrease of PH value.

Deciphering osteoconductive surface charge effects in sintered hydroxyapatite via piezoresponse force microscopy

Hydroxyapatite (HAp), a main constituent of the cortical bone, retains a large surface charge that plays a critical role in the regeneration of bones, whose source remains an enigma. Here, we used multi-eigenmode piezoresponse force microscopy via Pearson correlation and conducted multiscale material analysis to independently measure the piezoelectric coefficient and the surface charge density of HAp. Quantitative comparison was conducted between the measured surface charge density and the conceptual values computed for all possible candidates of the surface charging such as piezoelectricity, chemically induced surface charging, flexoelectricity, and defect dipole moment. The results presented Ca2+ ions released during local calcium orthophosphate phase transition at the surface as the main source of the surface charging. Further analysis showed that the ion concentration was modulable using a charged conductive tip, implying that the interstitial ions are mobile within the HAp matrix. In this work, we developed a novel method to measure the effective surface charge density of arbitrary material systems, thereby facilitating the investigation of biophysical phenomena related to surface electromechanics. Most importantly, the research promotes a new understanding of the origin of osteoconduction observed in past works conducted on HAp.

Highly Rectifying Fluidic Diodes Based on Asymmetric Layer-by-Layer Nanofilms on Nanochannel Membranes.

Nanochannel-based fluidic diodes display ion selectivity and ion current rectification (ICR), which may prove to be important in energy-harvesting devices and biosensors. This paper reports asymmetric functionalization of the outer surface of a flexible nanochannel polymer membrane to create fluidic diodes that give ICR. Layer-by-layer (LbL) adsorption with cross-linking of only the underlying part of the polyelectrolyte nanofilm leads to a porosity step across the film. The combination of a high effective surface charge density and the porosity step in the film leads to a remarkable maximum ICR factor of ∼200 with a pH gradient across the film. Incorporation of pH-sensitive polyelectrolyte components enables the ICR factor to increase an order of magnitude on going from pH 8 to pH 3. Moreover, the coated membrane shows excellent anion selectivity. Thus, LbL adsorption with partial cross-linking provides a simple method for creating coated nanochannel membranes that serve as pH-responsive ionic diodes for potential chemical/biosensors.

Studying properties of corona electret based on compositions of polylactic acid and bentonite

The study focuses on the composites of polylactic acid and bentonite as well as corona electrets based on them. The paper explores rheological, electrical, physical and mechanical properties of the composites as well as the effect of corona charging on their properties. Differential scanning calorimetry was used to assess transition temperatures in the compositions. Surface potential, electric field strength and effective surface charge density were measured. Optimal performance was shown by the compositions of polylactic acid with 4% bentonite.

Rationally patterned electrode of direct-current triboelectric nanogenerators for ultrahigh effective surface charge density

As a new-era of energy harvesting technology, the enhancement of triboelectric charge density of triboelectric nanogenerator (TENG) is always crucial for its large-scale application on Internet of Things (IoTs) and artificial intelligence (AI). Here, a microstructure-designed direct-current TENG (MDC-TENG) with rationally patterned electrode structure is presented to enhance its effective surface charge density by increasing the efficiency of contact electrification. Thus, the MDC-TENG achieves a record high charge density of ~5.4 mC m−2, which is over 2-fold the state-of-art of AC-TENGs and over 10-fold compared to previous DC-TENGs. The MDC-TENG realizes both the miniaturized device and high output performance. Meanwhile, its effective charge density can be further improved as the device size increases. Our work not only provides a miniaturization strategy of TENG for the application in IoTs and AI as energy supply or self-powered sensor, but also presents a paradigm shift for large-scale energy harvesting by TENGs.

Investigation of electret and filtering properties of polypropylene-based nonwoven fabrics and its composites with 2 vol% of silicon dioxide inclusions

The electret characteristics of polypropylene (PP) nonwoven fabrics processed by melt-blowing and its composites with 2 vol% of silicon dioxide (SiO 2 ) are studied in this work. The surface potential V e , effective surface charge density a ef and electric field strengths E as well as their filtration efficiency are reported. Improvement of electret properties of PP filled with SiO 2 in comparison with the samples without filler is observed. It is seen that PP samples poled in a corona discharge show lower filtration rate than the non-poled ones. The electret nonwoven fabrics with improved filtration characteristics are obtained on the basis of PP with SiO 2 inclusions.

Specific Ion Effects on the Colloidal Stability of Layered Double Hydroxide Single-layer Nanosheets

The surface charge properties and aggregation behavior of positively charged Mg-Al-NO3 layered double hydroxide (LDH) single-layer nanosheetsdispersed in water were investigated in the presence of K+salts with different mono-, di-, and trivalent anions, using electrophoresis and dynamic light scattering techniques. An increase in the salt concentration can significantly decrease the effective surface charge density (σeff) of LDHs, leading to the aggregation of nanosheets. The critical coagulation concentration (CCC) or ionic strength (CCIS) of salts for nanosheets significantly decreases with an increase in the valence of anions. Specific ion effects, with a partially reverse Hofmeister series, are observed. Based on the Stern model and the DLVO theory, the relationship of CCC with σeff and the ionic valences of salts (zi) is theoretically analyzed, which can accurately describe the dependence of CCC on the σeff and zi, but cannot explain the origin of specific ion effects. To explore the origin of specific ion effects, a correlation between CCIS and the specific adsorption energy (Esc) of anions within the Stern layer is developed. Especially, an empirical relationship of Esc with the characteristic physical parameters of anions is proposed. Our model can accurately predict the CCISs of at least monovalent anions and divalent anions (CO32- and SO42-), demonstrating that the specific ion effects observed can be attributed to the differences in ionic size, polarizability, and hydration free energy (or the formation capacity of anion-cation pairs) of different anions. This work not only deepens the understanding of specific ion effects on the colloidal stability, but also provides useful information for the potential applications of LDH single-layer nanosheets.

How do ions contribute to brine-hydrophobic hydrocarbon Interfaces? An in silico study

HypothesisThe saltwater-oil interface is of broad implication in geochemistry and petroleum disciplines. To date, the main focus has been on the surface contribution of polar, heavy compounds of crude oil, widely neglecting the role of non-polar hydrocarbons. However, non-polar compounds are expected to contribute to characteristics of oil-brine interfaces. MethodologyUtilizing molecular dynamics simulation, we aim to characterize ion behavior adjacent to hydrophobic organic phases. Concerning natural environments, NaCl, CaCl2 and Na2SO4 electrolytes at low (5 wt%) and high (15 wt%) concentrations were brought in contact with heptane and/or toluene which account for aliphatic and aromatic constituents of typical crude oils, respectively. The reproduced experimental data for interfacial tension, brines density and ions’ diffusivities adequately verify our molecular calculations. FindingsIons accumulate nearby the intrinsically charge-neutral oil surfaces. A disparate surface-favoring propensity of ions causes the interfacial region to resemble an electrical layer and impose an effective surface charge onto the oil surface. Despite absence of any polar site, the effective surface charge density is hydrocarbon-dependent, with the highest and lowest values observed for toluene and heptane interfaces, respectively. Due to accumulation of toluene molecules nearby the brines, the interfacial characteristics of heptol (toluene-heptane mixture) is comparable to that of the toluene phase.

Criteria for colloidal gelation of thermo-sensitive poly(N-isopropylacrylamide) based microgels

HypothesisSuspensions of the poly(N-isopropylacrylamide) (PNIPAM) based temperature(T)-sensitive microgels can undergo colloidal gelation forming a three-dimensional sparse network-like structure in the hydrophobic and shrunken state of T > T* (T*: volume transition temperature), despite their considerably low particle volume fractions (<0.2). The effective surface charge density is expected to be a key factor governing the colloidal gelation and gel modulus. ExperimentsThe combined analysis of the viscoelasticity and electrophoretic mobility (EPM) was performed varying systematically pH and ionic strength (I). The microgels containing the extremely small content of electrolyte (0.1 mol%) with the T* and swelling degree being insensitive to pH and I were employed to facilitate the exclusive analysis of their effects on colloidal gelation. FindingsThe results unambiguously reveal (1) that the gelation requires the adequate suppressions of the interparticle charge repulsion, and (2) that a reduction in the interparticle charge repulsion results in an increase in gel modulus by several orders of magnitude. The long-term linear creep behavior show that the colloidal gels are identified as a viscoelastic fluid with a long relaxation time and a high viscosity whereas they behave elastically at relatively short timescale in conventional oscillatory tests.

Application of the Electret in Alpha Radiation Sensor to Measure the Concentration of Radon in Selected Ambient Conditions

A novel electret sensor, made from a polytetrafluoroethylene (PTFE) foil, is proposed for monitoring of radon concentration in closed spaces. A triode air system was implemented to form the electret foil. In order to calibrate the electret sensor, the ionization chamber with the electret embedded at its bottom, was placed in an emanation chamber containing radon of known concentration. The change in the effective charge density of the electret resulting from the interaction of the electret sensor with ionizing radiation, make it possible to determine the degree of its response to the radon concentration in the examined environment. The electric charge of the electret sensor was evaluated at the beginning and at the end of the concentration measurement using a field chopper. The measurement of the value of the electric field between the electret and the reference electrode directly corresponds to the effective surface charge density of the electret. The measuring system was calibrated in values of the electret equivalent voltage of Uz. In order to assess the electret properties, both the thermostimulated charge relaxation (TSQ) method and the thermostimulated discharge current (TSDC) method were used to evaluate corresponding parameters. These investigations led to the optimization of the electret formation process for the maximum sensitivity to the effects of alpha particles resulting from radon radioactive decay.

About nature of the effective surface charge on InAs crystals transformation during anode oxide layer growing

Dynamics of changes in fluorine atoms distribution through grown anodic oxide layer thickness and the effective surface charge on InAs crystals under such layers has been studied. Anodic oxidation was performed in alkaline electrolyte with fluorochemical additive component in galvanostatic mode at anode current densities 0.05 or 0.5 mA·cm−2. The layers thickness in boundes 32—51 nm varied by electrodes final voltage setting in range 15—25 V. The layer thickness and refractive index was measured by ellipsometric method, and distribution of fluorine atoms through thickness — by photoelectron−spectroscopy method, combined with ion etching. At the same time, based on grown layers there were produced MIS structures, and from calculation of theirs capacitance−voltage characteristics are determined effective surface charge and surface states density, corresponding to different layer thicknesses.Main results are reduced to the facts during layers growing despite of anodizing current density comes their sealing, the profile of fluorine atoms distribution shifts towards InAs, positive effective surface charge gradually decreases from 3.6 · 1011 to 2.0 · 1011 cm−2 at surface states density in (6—7) · 1011 eV·cm−2 range for all cases. Based on comparison of these data and theoretical concepts of MIS structure charge construction, there was made a conclusion about gradual built−in charge distancing from the border with InAs in the process of growing anodic oxide layer, which explains observed effective surface charge decrease during layer thickness increasing. This results indicates that the layer growth rate exceeds the built−in charge displacement rate towards InAs.